1. Field of the Invention
This invention pertains to fiber optics and to a device that impresses information on an optical carrier to be transmitted by an optical fiber. More particularly it relates to electro-optical modulator devices using Mach-Zehnder-type modulators in which the first and second branches of the waveguide have reverse polarity and which further have one or more of the following characteristics: 1) the electronic signal driving voltage of the modulator is substantially lowered, 2) the electronic signal propagation velocity is close to the velocity of the optical carrier, 3) the electric field of the electronic signal is uniform and produces a net overlap of unity with the optical wave.
2. Background of the Invention
Mach-Zehnder (M-Z) devices in which the first and second wave guide branches have reverse polarity was first demonstrated in U.S. Pat. No. 5,267,336 for electrode-less and lumped electrodes in push-pull configurations. To avoid the resistance-capacitance time constant, typically called the RC time constant, various workers have provided electrodes that operate in a transmission line type configuration rather than capacitor-type lumped electrode configuration. U.S. Pat. No. 6,334,008, all of which is incorporated herein by reference, illustrates the use of transmission-type electrodes with two waveguide branches having reverse polarity. Unfortunately, because the positive and negative electrodes carrying the electronic signal are arranged on the same face of the electro-optic crystal, the full effect of electronic signal electrical field is not experienced by the optical carrier. Rather it is only that portion of the electric field of the electronic signal that interacts with the optical carrier. Further, because the angular distribution of the electric field electronic signal is not uniform as it passes through the optical wave guide branches, the optical signal experiences a non-uniform electronic signal in each plane across (perpendicular to) the optical path in the optical wave guides. Such electric field distortion in the optical waveguides leads to decreased modulation bandwidth. Further because the traveling wave of the electronic signal travels within the electrode as well as in the optical waveguide substrate, it experiences a material refractive index (electronic signal refractive index) that is different from that experienced by the optical signal with results in differing propagation velocities for the electronic and optical signals. This results in further distortion of the optical signal and decreased modulation bandwidth.
Several references, U.S. Publication Nos. 2001/0004410 and 2001/0007601, and U.S. Pat. No. 6,120,597 have improved the match between a transmission-type electronic signal and the optical signal velocities by thinning the waveguide substrate and/or using adhesives and fixing substrates that bring the electronic signal refractive index closer to that of the optical signal. However, the electronic signal electrodes are placed on the same face of the optical waveguide substrate resulting in the less than optimal interaction of the electronic electric field with the optical signal noted above.
In view of the above deficiencies and in order to improve the operation of an MZ type optical modulator, it is an object of the present invention to increase the electric field strength across the waveguide portion of the waveguide substrate.
It is an object of the present invention to produce a net overlap of unity between the optical signal wave and the electric field of the electronic signal.
It is an object of the present invention to produce a M-Z device with increased phase modulation which increases intensity modulation.
It is an object of the present invention to provide better velocity matching between the electronic signal and the optical signal.
It is an object of the present invention to improve impedance matching with the electronic signal drive circuitry.
It is an object of the present invention to reduce electronic signal voltage (Vxcfx80) as much as possible.
It is an object of the present invention to vary the refractive index in two waveguides of a M-Z device by using a single applied voltage.
It is an object of the present invention to vary the refractive index in two waveguides of a M-Z device without the use of a voltage inverter.
It is an object of the present invention to vary the refractive index in two waveguides of a M-Z device while reducing Vxcfx80 below 80% of a single waveguide modified M-Z device while using a single driver with a single applied voltage without the use of an inverter circuit.
It is the object of the present invention to vary the refractive index in the two waveguides of a M-Z device in equal and opposite amounts to afford chirp free operation.
It is the object of the present invention to vary the refractive index of the two waveguides of a M-Z in arbitrary amounts to adjust the chirp factor of the device.
It is the object of the present invention to increase the transmission speed of the system by adjusting the chirp of the device.
It is the object of the present invention to increase the distance of the transmitted signal by adjusting the chirp of the device.
It is the object of the present invention to reduce the size of the electro-optical modulator.
It is the object of the present invention to increase the manufacturing yield from a starting wafer as a result of its reduced size.
It is an object of the present invention to improve the overall device stability.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail. It is contemplated that variations in procedures may appear to a person skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention.
To meet this objects, the present invention features 1 waveguide channels formed in a crystal substrate in which two of the waveguide channel branches have reverse polarity relative to each other. More specially the waveguide substrate have the following waveguide sections formed in a face of the waveguide substrate: 1) an input waveguide section for receiving an optical signal, 2) an input branching waveguide section for dividing the optical signal into a first portion and a second portion, 3) a first branch waveguide for transmitting the first portion of said optical signal; 4) a second branch waveguide for transmitting the second portion of said optical signal and having reverse polarity to that of the first branch waveguide, and 5) an output branching waveguide section for combining the first portion of the optical signal from the first branch waveguide and the second portion of the optical signal from the second branch waveguide to form an optical output signal. An electronic signal electrode and an electronic ground electrode are placed in proximity with the first branch waveguide and the second branch waveguide and oriented so as to produce an electric field in the first and second branch waveguides that affords an optical output signal that is proportional to the electric field. As used here the xe2x80x9cterm proximity with first and second branch waveguides means that the electrodes can be placed near to or in contact with the branch optical waveguides and includes embodiments in which the electrodes are placed on opposite sides of the waveguide substrate. In addition to the requirement that two of the branch waveguides have reverse polarity with respect to each other, the optical modulator further must have at least one of the following: 1) the electronic signal electrode and the electronic ground electrode must be placed in proximity with opposite faces of the waveguide substrate, 2) a portion of said waveguide substrate must be removed to afford a reduced electronic signal refractive index, and 3) the waveguide substrate must have a fixing substrate attached to it with the refractive index of the fixing substrate lower than said refractive index of the waveguide substrate. Any particular embodiment must have at least one of the three waveguide substrate arrangements. However, any particular embodiment is not limited to one of the arrangements and may include either one or two of the remaining the remaining arrangements. Portions of said waveguide substrate are removed by forming apertures in said waveguide substrate outside of the optical waveguide sections. These portions can be in the form of apertures or grooves. When a fixing substrate is attached to a face of the waveguide substrate it must have a refractive index lower than that of the waveguide substrate. The fixing substrate may be attached to the waveguide substrate directly or by using an adhesive. When an adhesive is used to join the fixing substrate to the waveguide substrate, the adhesive must have a refractive index lower than that of the waveguide substrate. The advantage of removing one or more portions of the waveguide substrate is that this affords a reduced electronic signal refractive index which in turn allows the electronic signal and the optical signal to propagate in the modulator at nearly the same velocity thereby affording. In the case where electrodes are formed on opposite faces of the waveguide substrate, neither the fixing substrate nor the adhesive serve to reduce the electronic wave refractive index. As such, any adhesive or fixing substrate may be used without regard to their refractive index properties. Typically the electronic signal and ground electrodes are formed in a transmission-type arrangement that accommodate a traveling wave electronic signal. In such a configuration, the electronic signal and ground electrodes are impedance matched with a load placed in parallel with said electronic signal and ground electrodes.